1. Field of Invention
The present invention relates to a disposable electrode strip which is easy to produce and can detect a concentration of uric acid in a liquid sample, its method for producing the same and its use. More specifically, the present invention relates to a non-enzymatic disposable uric acid electrode strip modified by a water soluble redox electron mediator which accurately detects the concentration of uric acid, avoids any interference caused by other components in the liquid, and is suitable for household use.
2. Description of the Related Art
Uric acid, a final product of the metabolism of purine, is mostly excreted from human body through the kidneys in the form of urine. The concentration of uric acid in blood increases when the source of uric acid increases or the kidney malfunctions. Hyperuricemia is a symptom when the uric acid concentration is above 7 mg/dl. Uric acid is hard to dissolve in blood and will crystallize when supersaturated. The uric acid crystallites deposit on the surface of skin, in joints, and especially in toes and results in gout. The analysis of the uric acid concentration in blood helps to diagnose gout. In addition to gout, hyperuricemia is connected with lymph disturbance, chronic hemolytic anemia, an increase of nucleic acid metabolism and kidney malfunction. High caloric foods and alcohol as well as disturbances of organs and tissues are the main causes of hyperuricemia and even gout. Harm can be prevented and reduced by an early diagnosis and by monitoring. A simple and inexpensive detecting system helps patients to detect the uric acid concentration on their own.
The quantitative analysis of uric acid is basically divided into weight measurement, titration, reduction and enzymatic method. The weight measurement and the titration involve purifying the precipitates of uric acid with magnesium, ammonium or copper and measuring the weight of the precipitates. The weight measurement and the titration are complex but not accurate and therefore they are not suitable in analyzing uric acid. General clinical biochemical analysis so far adopts a reduction or an enzymatic method to detect uric acid. The introduction of reduction and the enzymatic method are in the following paragraphs.
(1) Reduction
The main reaction and theory are as follows: ##STR1##
The uric acid undergoes an oxidation reaction with phosphotungstate and produces tungsten blue in an alkaline solution with a pH value in the range of 9 to 10. The content of uric acid is measured by spectrophotometer at 660.about.740 nm. The demerits of the method include: 1) some compounds similar to uric acid and ascorbic acid contained in the blood sample affect the test accuracy; 2) the operation is complex, needs lots of agents which are hard to keep, and should be operated by professionals; 3) the sample must be de-protein pretreated; and 4) the necessary equipment is expensive.
(2) Enzymatic method
The enzymatic method detects uric acid by optical colorimetry and electrochemistry and is classified into uricase-ultravialet absorption, uricase-peroxidase, uricase-catalase and uricase-electrode methods, wherein the former three methods make use of the color of reaction products and quantitatively detect uric acid of products by colorimetry. The automatic bio-analyzers used in central bio-laboratories of hospitals detect uric acid by optical colorimetry. The blood sample should be pretreated to be serum or plasma first. The merits of the automatic bio-analyzers reside in mass detecting, automation and quickness. However, an automatic bio-analyzer cannot be applied in household detecting because it requires professionals to operate, is expensive, and it is particularly hard to store the detecting agents.
The uricase-electrode method detects uric acid by electrochemistry. The electrodes can be divided as enzymatic and non-enzymatic. The former produced by a complex production process is hard to store and thus is only suitable for research. The related prior research of the latter were as follows:
Park G, Adams R N, White W R (Anal. Lett., 1972; 5:887) detected uric acid by measuring current signals produced by an electrochemical oxidation of uric acid on a carbon-based electrode. The reaction system was not accurate because of the interference resulting from ascorbic acid in an acid solution. Several researches tried to avoid the influence of interference and to improve the specificity to uric acid.
For example, X. Cai, K. Kalcher, C. Neuhold and B. Ogorevc (Talanta, 1994; 41:407.about.413) placed a carbon paste electrode in an alkaline solution and apply 1.4V vs. SCE for 40 seconds to be anodized. This method forced the oxidation potential of uric acid to shift to around 0 mV vs. SCE, increased the response current and distinguishes uric acid from ascorbic acid. The detectable linear ranges were from 3.times.10.sup.-8 to 2.4.times.10.sup.-4 M (5.times.10.sup.-4 to 4 mg/dl) and the detecting limit was 1.2.times.10.sup.-8 M (2.times.10.sup.-3 mg/dl). The manufacturing process was time-consuming and complex and not suitable for production on a large scale. The electrode could not be easily operated. Most important of all, the linear ranges were out of the normal dianostic range of 2.about.14 mg/dl.
Ai-min Yu, Hai-li Zhang and Hong-yuan Chen (Analyst, 1997; 122:839.about.841) modified the surface of glassy carbon electrode by electro-polymerization of polyglycine. The modified electrode could distinguish potential of uric acid (0.45 mV vs. SCE) from ascorbic acid (0.30 mV vs. SCE) and increased the response current of uric acid after redox reaction. The detectable linear ranges were from 5.times.10.sup.-8 to 4.5.times.10.sup.-6 M (8.4.times.10.sup.-4 to 7.5 .times.10.sup.-2 mg/dl) and the detecting limit was 5.times.10.sup.-9 M (8.4.times.10.sup.-5 mg/dl). The urate sample was prepared by dissolving uric acid in a 0.1M phosphate buffer solution with a pH value of 7.0. The electrode manufacturing process was time-consuming and complex and not suitable for mass production. The electrode costs high, and could not be used as a disposable strip. Besides, the linear ranges were also out of normal diagnostic ranges.
Jyh-Myng Zen and Jen-Sen Tang (Anal. Chem. 1995; 67:1892.about.1895) modified the glassy carbon electrode by Nafion/Ru.sub.2-x Pb.sub.x O.sub.7-x (ruthenium oxide pyrochlore) and detected uric acid by Osteryoung square-wave voltammeter. The redox potential of uric acid was +0.65 V vs. Ag/AgCl where the redox potential of ascorbic acid was 0.5 V vs. Ag/AgCl. The electrode would not be interfered with by the ascorbic acid unless the concentration of ascorbic reached to 20 mg/dl level. The electrode worked in the pH value of 1. The detectable linear ranges were from 7.5.times.10.sup.-5 to 5.times.10.sup.-7 M (8.4.times.10.sup.-3 to 1.26 mg/dl) and the detecting limit was 1.1.times.10.sup.-7 M (1.8.times.10.sup.-3 mg/dl). The electrode shared the demerits mentioned above, and also can not be used as a disposable strip.
Markas A. T. Gilmort and John P. Hart (Analyst, 1992; 117:1299.about.1303) detected uric acid by a screen printing carbon-based electrode modified by Nafion containing L-ascorbic acid oxidase. The modification decreased the interference of ascorbic acid and increased the specificity to uric acid. The optimum reaction condition was at pH value of 5.5 and operation voltage of +0.4 V vs. SCE. The detectable linear ranges were from 5.08.times.10.sup.-6 to 1.51.times.10.sup.-4 M (8.5.times.10.sup.-2 to 2.54 mg/dl) and the detecting limit was 2.54.times.10.sup.-4 M (4.3.times.10.sup.-2 mg/dl). The tolerance concentration of ascorbic acid was below 0.53 mM (9.34 mg/dl) so as not to interfere with uric acid. The electrode could be mass-produced but the cost was high and several manufacturing steps are involved. The detectable linear ranges were also not within general diagnostic ranges(2.about.8 mg/dl). The sample should be pretreated and thus the electrode cannot be operated at home.